Update for M3

Hey guys,

This is an update for M3. The main thing I changed in this commit is some of the HttpClient stuff: https://github.com/timothyarmstrong/discovery_api_dart_client_generator/commit/f6694ba0c14e94471c143dd3ed0baa7593fc579a#L3L19

The generator seems to hang after creating the libraries though. Any idea why that would be?

(I also updated some deprecated stuff).